Method for processing naphtha

ABSTRACT

Process for the work-up of naphtha, wherein a) naphtha or a stream produced from naphtha in a pretreatment step is separated in a membrane unit into a stream A which is depleted in aromatics and a stream B which is enriched in aromatics, with the aromatics concentration in stream A being from 2 to 12% by weight (step a), b) at least part of the substream A is fed to a steam cracker (step b), c) at least part of the substream B is fed to a unit in which it is separated by means of a thermal process into a stream C which has a lower aromatics content than stream B or a plurality of streams C′, C″, C′″ . . . which each have lower aromatics contents than stream B and a stream D which has a higher aromatics content than stream B or a plurality of streams D′, D″, D′″ . . . which each have higher aromatics contents than stream B (step c), and d) the stream C or at least one of the streams C′, C″, C′″, in each case in part or in its entirety, is added to the feed to the steam cracker or fed into the steam cracker itself (step d).

The present invention relates to a process for the work-up of naphtha,wherein

-   a) naphtha or a stream produced from naphtha in a pretreatment step    is separated in a membrane unit into a stream A which is depleted in    aromatics and a stream B which is enriched in aromatics, with the    aromatics concentration in stream A being from 2 to 12% by weight    (step a),-   b) at least part of the substream A is fed to a steam cracker (step    b),-   c) at least part of the substream B is fed to a unit in which it is    separated by means of a thermal process into a stream C which has a    lower aromatics content than stream B or a plurality of streams C′,    C″, C′″ . . . which each have lower aromatics contents than stream B    and a stream D which has a higher aromatics content than stream B or    a plurality of streams D′, D″, D′″ . . . which each have higher    aromatics contents than stream B (step c), and-   d) the stream C or at least one of the streams C′, C″, C′′, in each    case in part or in its entirety, is added to the feed to the steam    cracker or fed into the steam cracker itself (step d).

Petroleum fractions referred to collectively as naphtha generallycontain considerable concentrations of aromatics, in particular benzene,toluene and xylenes, in a total concentration of usually from 5 to 50%by weight. If the naphtha is processed in a steam cracker with the aimof producing ethylene, propylene and further high-value hydrocarbons,the abovementioned and possibly other aromatics remain essentiallyunreacted there. In this use of the naphtha, the aromatics thereforerepresent an undesirable ballast which, for a given amount of desiredproducts, leads in the steam cracker to an increase in the size of manycomponents of the steam cracker and also to an increase in the operatingcosts, especially those incurred as a result of heating and recoolingthe feed or product stream.

In Desalination 149 (2002), 29-34 (Meindersma, de Haan), it is proposedthat naphtha be fed to a membrane unit in which aromatics areselectively removed from the naphtha before it is fed into the cracker.The membrane unit described there is equipped with aromatics-selective(see below for definition) zeolite membranes and is designed so that afeed stream having an aromatics content of 10% is separated intosubstreams having aromatics contents of 2 and 98%, respectively.

These comparatively high purities of the substreams and the fact thatthey are produced exclusively by means of membranes lead, in the variantdescribed, to very high costs, especially because of the large membranearea required.

It is an object of the present invention to provide a more efficientprocess for the work-up of naphtha in a steam cracker, in which the feedto the steam cracker is not heavily laden with largely inert aromatics.

Naphtha is generally a mixture comprising paraffinic, aliphaticalicyclic, aromatic and olefinic C₄-C₁₂-hydrocarbons boiling in therange from 30° C. to 230° C., especially from 30° C. to 180° C. and inparticular from 30° C. to 160° C. Such mixtures are usually obtained bydistillation from crude oil, crude oil fractions, from the feed tocatalytic reforming plants, from the reformate or the reformatefractions from the catalytic reforming plant, and fractions from thermaland catalytic cracking plants, with a number of these measures alsobeing able to be combined. Naphtha can also be produced by mixing(blending) various crude oil fractions, reformate, reformate fractionsor fractions from thermal or catalytic cracking plants.

The aromatics to be separated off are usually aromaticC₆-C₁₂-hydrocarbons and mixtures thereof. The aromatics are usuallyessentially benzene, toluene or xylenes or mixtures thereof.

The concentration of aromatics in the naphtha used or in the streamproduced from naphtha in a pretreatment step is from 8 to 50% by weight,preferably from 8 to 20% by weight, particularly preferably from 10 to15% by weight.

The feed stream can have been pretreated in a hydrogenation plant inorder to reduce the sulfur (S) and/or nitrogen (N) compounds and/or tosaturate the olefins.

The pretreatment step mentioned under a) preferably comprises adistillation or rectification which, for example, separates the naphthainto two streams having different aromatics contents, of which thestream which is richer in aromatics is fed to the membrane unit and thestream which has a lower aromatics content is fed directly to thecracker or added to the feed stream to the cracker. This offers theadvantage that, at a given amount of aromatics in the naphtha fed to theoverall process, the concentration of aromatics in the membrane unit isincreased, which leads to an increase in the transmembrane flux.

The membrane unit is configured and operated so that the aromaticsconcentration in the stream A is from 2 to 12% by weight, preferablyfrom 4 to 10% by weight. The aromatics concentration in stream B isgenerally from 10 to 90% by weight, preferably from 20 to 60% by weight.

The membrane unit referred to under a) preferably comprises at least onearomatics-selective membrane. In this context, the term“aromatics-selective” means that the total concentration of aromatics inthe permeate of a membrane apparatus equipped with the membrane ishigher than that in the retentate. Such membranes are known per se tothose skilled in the art; descriptions may be found, for example, inU.S. Pat. No. 2,930,754 (Stuckey), U.S. Pat. No. 5,128,439 (Sartori etal.) and EP 583,957 (Sartori et al.).

A measure of the aromatics selectivity is the separation factor αapplicable to a particular mixture of an aromatic and a nonaromatic,which is defined as follows:α(aromatic/nonaromatic)=(C _(A,P) /C _(NA,P))/(C _(A,F) /C _(NA,F))where

-   C_(A,P)=concentration of the aromatic on the permeate side,-   C_(NA, P)=concentration of the nonaromatic on the permeate side,-   C_(A,F)=concentration of the aromatic on the feed side,-   C_(NA,F)=concentration of the nonaromatic on the feed side,    where the feed side is the side of the membrane where the mixture to    be separated is introduced. The stated concentrations are, according    to the applicable definition, local, i.e. only at a defined point on    the membrane in a membrane apparatus; the separation factor α can be    determined experimentally by, for example, the ratio of feed mixture    flowing through a measurement cell equipped with a piece of membrane    to the area of the piece of membrane being selected so that the    composition of the feed mixture remains virtually constant while it    flows over the membrane.

The membrane used in the process of the present invention has an a value(toluene/n-octane) of from 1.5 to 100, preferably from 4 to 40,particularly preferably from 5 to 20.

The membranes used can, as one alternative, comprise polymers,particularly preferably polymers containing polar groups; examples ofsuitable polymers are polyacrylates, polyacrylic acid, polycarbonates,polyterephthalates, polyurethanes, polyamides, polyimides,polyetherimides, polyether ketones, cellulose derivatives, partiallyhalogenated polyolefins, e.g. PVC, or various sulfonated polymers andalso copolymers. When polymer membranes are used, it has been found tobe advantageous for them to be crosslinked by means of suitablereagents, because this can counter selectivity-reducing swelling of thepolymer.

The membranes can also consist of inorganic material such as microporouscarbon (produced by pyrolysis of organic polymers such as PP) orzeolites. Zeolites of the faujasite type, e.g. NaX or NaY, areparticularly useful here.

The membranes are preferably configured as integral-asymmetricalmembranes or as composite membranes in which the actual separation layereffecting the molecular separation has a thickness of from 0.05 to 100μm, preferably from 0.1 to 30 μm, and has been applied to one or moremesoporous and/or macroporous support(s).

The membranes can be used in the form of flat, cushion or capillaryelements or mono-channel tubular elements or multichannel tubularelements, as are known per se to those skilled in the art. In the caseof membrane elements having a tubular geometry, the separation layer ispreferably located on the inside of the tube.

The membranes are generally surrounded by one or more housingscomprising a polymeric, metallic or inorganic material, with theconnection between housing and membrane being formed by a sealingpolymer (e.g. elastomer) or inorganic material.

The membrane unit is preferably operated in the manner of apervaporation as is known per se to those skilled in the art, in whichthe mixture to be separated (feed) in liquid form is brought intocontact with the membrane and the stream passing through the membrane(permeate) is taken off in gaseous form. The temperature at which themixture to be separated is brought into contact with the membrane isfrom 20 to 400° C., preferably from 80 to 250° C. The pressure on thefeed side of the membrane is from 1 to 100 bar abs., preferably from 1to 20 bar abs. The pressure on the permeate side is from 1 to 2000 barabs., preferably from 10 to 1100 mbar abs., with the pressure on thefeed side always being higher than that on the permeate side. Thepermeate-side pressure is set by removing the permeate stream by meansof a vacuum pump and/or a compressor and/or by condensing the permeatestream at a temperature which leads to an intrinsic pressure of thepermeate mixture corresponding to the desired permeate pressure. In thecase of pervaporation, it can be advantageous to divide the membranearea required over a plurality of apparatuses and, for the purposes ofcompensating for the heat loss caused by the liquid-gas phasetransition, to install one or more heat exchangers between the membraneapparatuses.

However, the membrane unit can also be operated in the manner of a vaporpermeation as is also known per se to those skilled in the art. This isdistinguished from pervaporation by the feed being brought into contactwith the membrane in vapor form.

The membrane process can be carried out as a single-stage process, i.e.the permeate from a membrane apparatus or the combined permeate from aplurality of membrane apparatuses through which the feed flows insuccession or in parallel forms/form, without further treatment, thestream B which is enriched in aromatics and the part which has notpermeated through the membrane (retentate) forms, without furthertreatment, the abovementioned stream A which is depleted in aromatics.As an alternative, the membrane process can be a two-stage or multistageprocess in which the permeate from one stage is passed as feed to thefollowing stage and the retentate from this stage is mixed into the feedto the first-named stage. Such arrangements are known per se (cf., forexample, Sep. Sci. Technol. 31 (1996), 729 ff).

The unit specified under c) preferably comprises one or moredistillation steps using one or more auxiliaries. In this context, anauxiliary is a component whose addition to a mixture of an aromatichydrocarbon and an aliphatic or cycloaliphatic hydrocarbon alters therelative volatility of the two hydrocarbons, preferably in such a waythat the relative volatility of the aliphatic or cycloaliphatichydrocarbon increases compared to the aromatic hydrocarbon. Suitableauxiliaries are preferably organic compounds which contain polarfunctional groups such as hydroxyl, amino, ketone, ester, sulfonyl,sulfoxide or, particularly preferably, amide. Most preferred auxiliariesare N-methylpyrrolidone and N-formylmorpholine. The distillation step orsteps using one or more auxiliaries is/are preferably carried out in themanner of an extractive rectification as is known per se (cf. K.Sattler, Thermische Trennverfahren, VCH, 2^(nd) edition 1995, p. 146ff.).

The unit specified under c) can also comprise an extraction step usingan extractant to which the description of the auxiliary given in thepreceding paragraph likewise applies. It can also be present as amixture with water. If the unit specified under c) comprises anextraction step, the latter is advantageously combined with adistillation apparatus in which separation of extractant and aromaticsis carried out.

Such processes are known per se to those skilled in the art, cf., forexample, Hillis and Folkins in: Ullmann's Encyclopedia, Sixth Edition,Electronic Release, Chapter 6.

In a preferred embodiment of the process, the aromatics-enriched streamB from the membrane unit is subjected to a rectification which ispreferably operated in such a way that the top product has a higherconcentration of hydrocarbons having 5 or fewer carbon atoms permolecule than the stream B and the bottom product has a higherconcentration of hydrocarbons having 6 or more carbon atoms per moleculethan the stream B.

All or part of the bottom product is passed to step c).

All or part of the top product can advantageously be added to the feedto the steam cracker or be fed to the steam cracker itself.

In general, the aromatics concentration of the stream C which isrecirculated to the steam cracker as set forth in step d) or of thestream or streams C′, C″, C′″ . . . which is/are recirculated to thesteam cracker as set forth in step d) is from 0.01 to 5% by weight.

1. A process for the work-up of naphtha, comprising a) separating, in amembrane unit, the naphtha or a stream produced from the naphtha into astream A which is depleted in aromatics and a stream B which is enrichedin aromatics, wherein the aromatics concentration in stream A is from 2to 12% by weight, b) feeding at least part of the stream A to a steamcracker, c) feeding at least part of the stream B to a unit whereinstream B is separated by means of a thermal process into at least onestream C comprising a lower aromatics content than stream B and at leastone stream D comprising a higher aromatics content than stream B, andadding the at least one stream C to the steam cracker directly, as partof a feed stream, or a combination thereof.
 2. The process of claim 1,wherein the naphtha is subjected, prior to introduction into themembrane unit, to a pretreatment step comprising separating the naphthainto two streams comprising different aromatics contents, wherein thestream comprising a higher aromatics content is fed to the membrane unitand wherein the stream comprising a lower aromatics content is fed tothe steam cracker directly, as part of a feed stream, or a combinationthereof.
 3. The process of claim 1, further comprising rectifying thestream B such that the top product of the rectifying of the stream Bcomprises a higher concentration of hydrocarbons comprising 5 or fewercarbon atoms per molecule than the stream B and the bottom product ofthe rectifying of the stream B comprises a higher concentration ofhydrocarbons comprising 6 or more carbon atoms per molecule than thestream B, and wherein at least part of the bottom product is passed tostep c).
 4. The process of claim 3, wherein at least part of the topproduct from the rectifying of the stream B is added to the steamcracker directly, as part of a feed stream, or a combination thereof. 5.The process of claim 1, wherein the separation by means of a thermalprocess comprises at least one distillation.
 6. The process of claim 1,wherein the aromatics are essentially benzene, toluene, xylenes, or amixture thereof.
 7. The process of claim 1, wherein the aromaticsconcentration in stream B is from 10 to 90% by weight.
 8. The process ofclaim 1, wherein the stream C comprises an aromatics concentration offrom 0.01 to 5% by weight.
 9. The process of claim 4, wherein the atleast part of the top product from the rectifying of stream B is addedto a feed and the feed comprising the at least part of the top productfrom the rectifying of stream B is fed to the steam cracker.
 10. Theprocess of claim 5, wherein the at least one distillation reduces therelative volatility of an aromatic hydrocarbon in a mixture of anaromatic hydrocarbon and an aliphatic or cycloaliphatic hydrocarbon 11.The method of claim 1, wherein the at least one stream C comprises thestreams C′, C″, and C′″, each of which is added to the steam crackerdirectly, as part of a feed stream, or a combination thereof.
 12. Themethod of claim 2, wherein the separating comprises distillation. 13.The process of claim 1, wherein the aromatics are benzene, toluene,xylenes, or a mixture thereof.